Hitherto, structural members used in automobiles and the like have been manufactured by press-working a steel sheet having a desired strength. Recently, on the basis of a requirement for a reduction in the weight of automobile bodies, for example, a high-strength steel sheet having a thickness of about 1.0 to 4.0 mm has been desired as a steel sheet material. However, with an increase in the strength of a steel sheet, workability of the steel sheet decreases and it becomes difficult to work the steel sheet into a member having a desired shape.
Consequently, as described in Great Britain Patent Application No. 1 490 535, a method for manufacturing a structural member, the method being called “hot pressing” or “die quench,” has attracted attention in which a high strength is realized by working a heated steel sheet in a metal mold and simultaneously rapidly cooling the steel sheet. This manufacturing method has been practically used for manufacturing some members that require a TS of 1.0 to 1.5 GPa. In this method, since a steel sheet is heated to about 950° C. and is then worked at a high temperature, a problem in terms of workability in cold pressing can be reduced. Furthermore, this method is advantageous in that since quenching is performed with a water-cooled metal mold, the strength of a member can be increased by utilizing a transformation structure, and the amount of alloying elements added to the steel sheet material can be reduced.
However, in a hot-pressed steel sheet member described in GB '535, a surface hardness significantly decreases which may often result in a deterioration of wear resistance or the like.
It could therefore be helpful to provide a hot-pressed steel sheet member which has a TS of 980 to 2,130 MPa and in which a decrease in a surface hardness is small, a steel sheet for hot-press, and a method for manufacturing the hot-pressed steel sheet member. Note that, herein, the “TS” of a hot-pressed steel sheet member refers to a TS of a steel sheet constituting the member after hot pressing.